Higher density optical disks have been developed. For example, one standard magneto optical disk is an AS-MO (Advanced Storage Magneto Optical Disk), which has a memory capacity of approximately 6 GB on one side of a diameter of 120 mm. This magneto optical disk has grooves G and lands L provided alternately in the radial direction (the arrow direction Ra) to form tracks as shown in FIG. 7. Each track has areas on which to form multiple fine clock marks 70 at a regular interval in the circumferential direction of the magneto optical disk. Each track is divided into multiple segments by these areas. The multiple segments are either address segments 8 or data segments 9. A frame comprises an address segment 8 and 38 data segments 9. The data segments 9 is an area for the user to record data in a magneto optical recording system while the address segment 8 is an area in which to store address data such as a track address.
The recording areas of a magneto optical disk are sectioned into multiple bands B (zones) as shown in FIG. 8. Multiple frames are aligned in the radial and circumferential directions in each band. A cycle of frames 1 to 9 is repeatedly arranged in the circumferential direction as shown in FIG. 9. Multiple frames aligned in the radial direction have the same frame number (frame address) in each band B. Address data stored in the address segments 8 shown in FIG. 7 include frame, band, and track addresses.
The so-called wobbling system is used for recording the address data in the address segments 8, in which wobbling sections 80 are provided on one of the sidewalls of the grooves G. Here, the morphology of wobbling sections 80 is schematically shown in the figure. The same is true for the other attached drawings FIGS. 1 to 6, described later. Two wobbling sections 80a and 80b are provided in the disk circumferential direction on a single address segment 8 for the track addresses N, (N+1), or (N+2) of the grooves G among the address data for the purpose of accurate reading. These two wobbling sections 80a and 80b are provided on a pair of sidewalls separately in a staggered manner. With the staggered manner, when the magneto optical disk is tilted and, consequently, it is difficult to detect one of the two wobbling sections 80a and 80b, the other one can be detected and the address data is reliably obtained.
The push-pull technique is used for reading the address data described above. The push-pull technique is hereafter briefly described. First, as shown in FIG. 10, a laser beam focused by an objective lens 6 illuminates the ridged surface formed by the lands L and grooves G, producing plus and minus reflected/diffracted lights R1. Consequently, the objective lens 60 receives the reflected/diffracted lights R1 in addition to the direct reflected light R0. These returning lights are given to a two-division detector 61 having two light receiving areas 61a and 61b. The difference between signals SG1 and SG2 output from the detector 61 and correspond to the received light amounts at the two light receiving areas 61a and 61b, respectively, is calculated. This is a push-pull signal. The wobbling rate at the laser beam illuminated section among the wobbling sections can be determined based on the push-pull signal.
The magneto optical disk pattern having the address segments 8 and data segments 9 described above is formed by rotating a glass master disk with a photo-resist applied and, concurrently, moving a laser beam focused by an objective lens in the radial direction to expose the disk to light, and then developing it. During exposure, one laser beam is divided into two and one of the two beams is controlled to wobble while exposing the parts corresponding to the wobbling sections 80. In this way, a groove G wobbled on one of the sidewalls and not wobbled on the other sidewall is obtained.
However, the prior art has the following problems.
First, an AS-MO standard magneto optical disk has a track pitch of 0.6 μm. Around this pitch, the wobbling sections 80 can be properly formed on one of the sidewalls of the groove G by means of the two-laser beam technique described above. However, when an attempt is made to reduce the track pitch to as small as 0.3 μm for the purpose of increasing the data recording density, it is difficult to properly form the wobbling sections 80 on the groove G by means of the two-laser beam technique. This is because the two beams increasingly overlap each other and become substantially a single beam spot as the distance between their beam spot centers is reduced.
Secondly, an AS-MO standard magneto optical disk utilizes a red laser having a wavelength of approximately 650 nm. Conversely, in order to produce magneto optical disks with higher densities, it is desirable to use a blue laser having a smaller wavelength (for example a wavelength of approximately 405 nm) for minimizing the beam spot. However, the detector is less sensitive to the blue laser than to the red laser. There may be an increasing risk that the wobbling sections 80 are not accurately detected. Particularly, separate optical detectors are used for detecting magneto optical signals and for detecting servo signals in the optical detection system of a magneto optical disk device. Therefore, a smaller amount of light is used for detecting the wobbling sections 80, which tends to cause inaccurate detection. Furthermore, in order to improve the S/N ratio of magneto optical signals, a smaller amount of light should be used for the servo operation and a larger amount of light should be used for detecting magneto optical signals. This will enhance the tendency above.